1. Field of the Invention
The invention pertains to an electric machine having a stator with a laminated core and hub received in a central opening of the core and fixed against rotation relative to the core.
2. Description of the Related Art
Electric machines are generally known. They comprise a stationary stator, which is assembled from lacquer-insulated, electric sheet steel stampings and carries an electrical winding, and a rotor, which is supported so that it can rotate relative to the stator. Internal rotors are installed radially inside the stator, and external rotors are installed radially outside the stator. In the latter type of motor, it is known that the laminated core of the stator can be supported on a metallic hub made out of, for example, aluminum or an aluminum alloy, and the core can thus be supported so that it will not rotate under the effect of reaction torques acting on it during the operation of the electric machine. Cooling channels are also formed inside the hub; these channels are connected to a coolant circuit, as a result of which the lost energy which accumulates as heat during operation can be carried away. So that heat can be transferred effectively between the laminated stator core and the hub, it is especially important that the contact zone between the two components be properly made. In this zone, the opposing surfaces of the laminated core and the hub should be designed to fit precisely together. When the stator of the electric machine is fabricated, the hub is usually cast separately, and then at least the surface of the hub which will be in contact with the laminated stator core is given a final machining. Independently of that process, the laminated stator core is fabricated by stacking the sheet metal stampings. The hub is then connected to the core by a shrink-fit process.
The production method described above is able to fulfill the functional requirements in a satisfactory manner, but it suffers from the disadvantage of being both labor-intensive and cost-intensive.
In the case of electric motors with an internal rotor, the conventional approach is to install the laminated stator core inside an external motor housing, which serves simultaneously to support and to cool the stator. For this purpose, it is already known that shrink-fitting can be replaced by a process in which a layer of aluminum alloy is cast around the laminated stator core. This reduces the production costs considerably. The shrinkage of the aluminum as it cools ensures that the housing encloses the laminated stator core tightly and with good surface-to-surface contact; good heat transfer across the contact surface is thus also made possible. A hardened steel bushing, which is inserted into the stator bore before the casting process, serves to center and to hold the laminated core in the die-casting mold. This bushing is not removed until the aluminum housing has cooled completely, which prevents the laminated stator core from warping.
In the case of motors with an external rotor, it is also desirable to form the stator hub, which is located in this case radially inside the laminated stator core, by casting the metal directly into the laminated core. This cannot be done in a straightforward manner, however, because the radially internal aluminum hub is subject to visible shrinkage as it cools after casting, and therefore at least certain areas of the hub will separate from the laminated core. A tight-fitting connection between the hub and the laminated stator core sufficient to meet the requirements cannot therefore be obtained. Casting experiments conducted by the applicant have shown that gaps are formed between the laminated core and the aluminum hub, and thus a sufficiently good bond cannot be obtained.